Tokkai 2000-173638 published by the Japanese Patent Office in 2000 discloses a warm-up device for increasing warm-up efficiency when a vehicle fuel cell power plant is started. In this prior art technology, the target power plant has two fuel cell stacks. According to this technique, heated water is first circulated only to one of the fuel cell stacks when the power plant is started. After warm-up of this fuel cell stack is completed, heated water is circulated to the other fuel cell stack.
The driving force required by the vehicle changes with the running conditions or loading conditions. In a fuel cell power plant for vehicles provided with two or more fuel cell stacks, it is preferred from energy efficiency considerations to operate only some fuel cell stacks when the drive force required by the vehicle is small, and to operate all fuel cell stacks when the driving force required by the vehicle increases.